In applications such as optical computing and optical communications, information is often encoded as pulses of light. Since these systems require optical clock pulses to operate, a source of optical square waves, preferably fast rise time optical square waves, is required so that such systems can be implemented as an all optical system. At this time, most techniques for generating optical square wave pulse trains are active techniques relying on the use of external modulators, RF oscillators or electrical feedback schemes to, for example, control a diode optical source. These techniques are, however, relatively complicated and expensive, are difficult to control and involve electrical switching which limits available rise time and frequency. A simpler, passive, all optical system would, therefore, be preferable.
At this time, passive generation of near-square wave optical pulse trains has been demonstrated only using polarization switching of diode lasers. This technique is, however, limited to laser cavities in which the gain and loss are relatively insensitive to the state of polarization of the light. The rise time in such devices is also not as rapid as might be desired for true square wave operation and the contrast ratio between on and off intensities is lower than might be desired for some applications.
A need, therefore, exists for an improved, all optical generator for fast rise time square wave optical pulse trains, which generator is relatively simple and inexpensive to construct and relatively easy to tune and control. It would also be desirable in some applications if such generator could be constructed to be compact.